fuel_sharing_technology

8/7/2019 fuel_sharing_technology

1/4


2/4

19Energy News Issue 22

Fuel oil operation

Fuel oil operation is based on the use ofnormal fuel oil injection pumps and thesystem operates just like any diesel engine.

The engine can run for example on lightfuel oil, heavy fuel oil or crude oil as themain fuel (and as the pilot injection fuel),

without any changes to the fuel injectionsystem. Fuel oil operation is available

within the full range from zero to the ratedload of the engine.

GD operation

GD is the original gas-diesel operation,where gas is the main source of poweralthough pilot fuel oil injection ofapproximately 5% is used to ignite the

combustion. GD operation is availablewithin 30...100% of rated load, and thetransfer to or from GD operation can beexecuted at any point within 30...87.5% ofrated load.

Fuel sharing operation

The fuel sharing option is available within35...100% of rated load. The fuel sharesetpoint can be adjusted on-line from

WOIS (Wrtsil Operators InterfaceSystem). For example the operator may

want to run at 30% gas share at full engine

load and to achieve this the 30% setpoint isgiven and the fuel sharing mode is selected.If at any time the engine load is too

low for reaching the selected setpoint, thecontrol system will automatically adjust

the setpoint according to the minimumor maximum limit. If the load returnsto a level where the 30% setpoint can bereached, the system will bring the fuel share

back to the setpoint. The operator maychange the setpoint at any time duringoperation.

Fuel sharing is limited to a specifiedwindow to ensure safe operation. Thefollowing numbers refer to the limitingpoints indicated with circles in Figure 1:

1. The minimum gas share (gas amount)is 15% of the power generation at full load.This is a fixed limit which originates fromthe minimum gas injection volume; hence,

when the load is decreased, the lower limitbecomes proportionally higher (a higher %

value). The minimum gas share is definedin order to allow continuous operation onlyat sufficiently long injection durations toensure repeatable and linear operation ofthe gas injection valve.

2. The maximum gas share (gasamount) in fuel sharing mode is about80%, at 87.5% of the engines rated load.The reason for not allowing continuousoperation between GD operation andthe maximum limit is because the fuel oilpump curve contains an inverse operatingarea between the main and pilot injection

curves; inverse operation means thatincreasing the fuel rack position actuallydecreases the fuel injection amount.This area is passed over as a step whentransferring to GD operation. The transfer

area is illustrated in Figure 2.3. Minimum engine output in fuel

sharing is 35% of rated load, and this isthe point where the minimum and the

maximum fuel share limits meet. If theengine load goes below 35%, the system

will automatically switch over to fuel oiloperation. The switchover is executed as afast transfer (not as a gas trip), which meansthat the gas supply system is maintained inready status for some time to enable rapidreturn to fuel sharing, should the load levelsoon be restored above 35%.

The actual fuel sharing value is derivedfrom the actuator positions. To reachan accurate actual value, the controlsystem works internally with linearized

actuator signals which are calibrated tosite conditions and site fuels. The accuracyis best if the fuel heat value does notvary. However, the system is tolerant ofvariations and these do not cause problemsin operation.

The fuel sharing technology

The fuel sharing system is essentially acombination of the GD engine and a newcontrol system and control principle. Thesystem controls both the gas and the oilinjection simultaneously and allows both

to be used within the allowed operatingwindow limits. The new control system istightly integrated into the programmablelogic control (PLC) of the Wrtsilextended level automation system using the

Fig. 3 Fuel sharing control system.


3/4

20 Energy News Issue 22

Wrtsil standard platform.The fuel sharing system consists of

multiple control loops which are all rununder one central processing unit (CPU).

The gas and speed control configuration isshown in Figure 3.

The speed and load are controlledby a dedicated PID controller which isembedded in the main PLC software. Thespeed controller provides a large variety ofoptimized control methods such as speeddroop, kW and speed droop compensationcontrols (SDC).

The fuel oil and fuel gas amounts arecontrolled by a dedicated twin-driveractuator controller which is also embeddedin the PLC software. It receives the global

control level signal from the speed PID,and divides the signal to the two actuatorsaccording to the setpoints and operatingconditions.

The PLC control signals are connectedto the actuators, which transform the globalcontrol signals into fuel injection cycles foreach cylinder with the correct timing andduration. The fuel oil actuator is electro-hydraulic, the same as in conventionaldiesel engines. The gas actuator is anElectronic Rail Valve (ERV) system whichcontrols cylinder-specific injection valves

based on engine speed and position, and aglobal control signal from the PLC.In addition to the twin-driver

functionality, the embedded controlsinclude the gas supply pressure control,

which keeps the gas pressure within250...350 bar based on the gas injectionlevel. With small gas injection amounts(low fuel share or low output GD), thegas pressure is kept lower to extend theduration of the injection.

The high-pressure (HP) sealing oil

pressure control is tightly integrated to thegas pressure control. The sealing oil pressureis kept 20 bar higher than the gas pressure,

which means that the HP oil pressure tracksthe gas pressure at all times.

Last but not least, the system integratesthe sequencing of the gas supply valves andthe safety functionality. Without goinginto too much detail about the sequencing,the gas supply valves have the followingfunctions:a) The double gas supply main valves secure

the isolation of the gas source from theengine, should this be necessary.b) A slow filling system is used during start-up. The gas line is filled through a smallerbypass line to prevent sudden pressure

Fuels

Fig. 4 Embedded fuel sharing controls.

Fig. 5 The Dygoil power plant in Ecuador is equipped with two Wrtsil 16V32LNGD

fuel sharing generating sets.


4/4

21Energy News Issue 22

supply system. The customer needed apower plant that could run on both gas andcrude oil because the gas supply was notstable and oil field gas availability declinesover time as the watercut increases. Wrtsiltook up the challenge and developed thefuel sharing system for the two Wrtsil

16V32LNGD generating sets in a veryshort time. The plant was handed over forcommercial operation in March 2004. (Seethe previous article).

This project best shows the main marketniche for fuel sharing: associated gas inoil fields or refineries. However, the fuelsharing capability is not limited to this typeof application. The fuel system can be used

with gas that has a low methane number andit can be used with very low heat value gas(in fuel sharing, and partially in GD).

The fact that it combines a large variety

of fuels may also make the fuel sharingsystem the most economical solution incertain cases. Since the control system isnow integrated into the Wrtsil standardextended level automation system, gasconversion applications or future gasconversions are now much more attractivethan with the original GD system.

The main advantages of the fuel sharingsystem are:a) Flexible operation at variable gas/oilshares.b) Flexibility in fuel oils, for example light

fuel oil, heavy fuel oil and crude oil can beused as main and pilot fuel.c) The system can operate on a low methanenumber gas.d) The system can use low heat value gas,especially in fuel sharing.

How does it run?

During the development of the fuelsharing system, we were often asked onequestion: is it really possible? Perhaps theidea of combining so many different anddemanding fuel types into one system

sounds too good to be true.However, as the test runs in May 2003

and the commissioning runs in March 2004clearly show, the system really works, and

well too. The trends recorded in Figure 5and the identifications in Figure 6 show howthe system transfers seamlessly from oneoperating point to another.

Conclusion

The fuel sharing system has now taken itsplace in the Wrtsil power plant applicationportfolio as the solution for demanding fuel

gas applications. With SG, DF and GD/fuelshare systems Wrtsil has a comprehensiveportfolio that can cover a wide variety of gasapplications, and better serve the customerneeds.

Fig. 6 Fuel sharing test run on 21 May 2003.

Fig. 7 Identifications to Figure 5.

0. 35% load in fuel oil operation.

1. 35% load in fuel sharing

operation, 40% gasshare.

2. 35% load in gas diesel operation.

3. Automatic transfer to fuel oiloperation as load drops below

30%.







8. Transfer to gas diesel operation,

80% load.

9. Manually activated gas trip to fuel

oil operation.

variations in the system.c) The blowdown system is used forrecirculating the compressed gas back to thegas compressor inlet when the gas is shutoff from the engine.d) The double venting valves are used forflaring the residual gas from the gas piping

after blowdown.e) The gas safety system will execute a gas tripand shut off the gas supply to the engine.The main embedded control functions areshown in Figure 4.

Fuel sharing applications

The fuel sharing principle was originallytested on a Wrtsil 4R32GD in 1999 forLNG tanker applications. The test wasperformed manually and it showed thatthe system works but development was nottaken any further at that time.

In 2002 Wrtsil was contactedby Petroproduccin Oil GovernmentCompany, an Ecuador-based company,

with a specification which called for truefuel sharing capability for an oil field power

fuel_sharing_technology

Documents